Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). A MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MRI data may be acquired using a three-dimensional (3D) acquisition strategy, the most common of which is a rectilinear sampling that fills a 3D Cartesian grid with Fourier reciprocal space (i.e., “k-space”) data. The data may be collected with Nyquist frequency sampling to provide unique location encoding of the MRI signals and thereby prevent aliasing in the reconstructed images. The 3D data is spatially encoded using phase encoding along two perpendicular spatial directions (the y and z directions) and frequency encoding along the third (the x direction). Usually, the secondary phase encoding is referred to as “slice encoding,” to distinguish it from the primary phase-encoding. The resultant raw data fills a 3D k-space matrix which is then “reconstructed” using Fourier transformation techniques, resulting in a stack of two-dimensional images.
To reduce acquisition time for 3D imaging, “parallel imaging” techniques (also known as “partially parallel imaging”) may be used in which k-space is undersampled (i.e., the Nyquist criteria is not met) and the signals from multiple receiver coils are combined to provide aliasing free images. Other techniques, such as auto-calibrated (or self-calibrated) parallel imaging techniques, homodyne techniques, and zero-filling techniques may be used, all of which use non-uniform sampling in the ky-kz plane, i.e., while some portions of k-space may be fully sampled at the Nyquist frequency, other portions may be undersampled or not sampled at all. Techniques such as auto-calibrated parallel imaging techniques, homodyne techniques, and zero-filling techniques may be referred to as “variable density” techniques, in which k-space is not sampled with uniform density of acquired views throughout. Variable density techniques may also be used for motion artifact reduction schemes or other purposes.
MRI data is typically collected in frames that are referred to as “views.” Each view corresponds to a single ky and kz value, but contains data for the full range of kx values that are required to reconstruct an image. Multiple view-ordering schemes are known in the art for determining how ky, kz encoding is performed for each view. For example, in a “nested” view-ordering scheme, all of the views corresponding to one phase-encoding axis (kz, for example) are acquired before incrementing the value on the other phase-encoding axis (ky, for example). An “elliptical centric” view-ordering scheme replaces the two nested loops with a single loop that steps through ky, kz pairs according to their distance from the origin in the ky-kz plane. The choice of a view-ordering scheme often depends on how the imaged object or its corresponding magnetization is expected to change during the data acquisition. Views near the center of k-space have the strongest effect on the overall image appearance, because most of the k-space information about an object is contained near the center of k-space. It is desirable to obtain the low-frequency views near the center of k-space when the imaged object or its magnetization is in a preferred state, for example, when an imaged object is relatively motionless or when the magnetization of the imaged object has evolved such that image contrast between two tissues of interest is near its maximum.
For most practical 3D imaging applications, it is not possible to obtain all of the views necessary to reconstruct an image in a single, uninterrupted acquisition. It is therefore frequently necessary to acquire 3D MRI data using multiple “shots,” each of which acquires a subset of the total required views. In a multi-shot acquisition, each shot may be preceded by a triggering event, such as by the playing out of a magnetization preparation (e.g., an inversion RF pulse or a fat suppression RF pulse) or by the receipt of a cardiac or respiratory trigger. Views obtained during one particular time window of the shot may be preferred to other views. For example, it may be desirable to use cardiac gating techniques to time the MRI data acquisition to diastole, when the heart muscle is more quiescent. In this example, a cardiac gating pulse triggers the start of each shot and views acquired during diastole are preferred. In another example, a fat-selective inversion RF pulse may precede each shot and invert the longitudinal magnetization from fat. Views acquired in a window around the null point of fat are preferred because the signal from fat in the acquired data is at a minimum with respect to other views in the shot. View-ordering within each shot is usually determined such that the views having the most desirable characteristics are used to fill the central portions of k-space and such that signal modulations, e.g., that may be due to the evolution of magnetization throughout the shot, occur smoothly in k-space. For a multi-shot technique, most conventional 3D view-ordering schemes are not compatible with the need to arrange the views from each shot such that the more desirable views are encoded for the center of k-space. Accordingly, it would be desirable to provide a view-ordering strategy for a multi-shot 3D MRI data acquisition that encodes the more desirable views from each shot into the center of k-space and that is compatible with uniform parallel imaging, variable density (e.g., self-calibrated) parallel imaging, other variable density k-space sampling schemes and multi-shot pulse sequences having a transient signal, i.e., signal varies from shot to shot.